The rolling of threads has become accepted as the method for producing superior threads for externally-threaded fasteners. However, an unfortunate characteristic of rolled threads is the relatively long runout at the inner end of the thread where the thread contour is not complete. During the thread rolling process, the fastener blank is rolled between two opposed dies having ridges complementary to the thread to be produced. These ridges run out at one edge of the die, which is the location where the inner end of the thread is formed. Inasmuch as the ridges are at a shallow acute angle with respect to the die edge, a feather edge is produced on these ridges which is susceptible to breakage. Because of this, the ridges of the die are tapered as the die edge is approached, becoming more shallow at the edge to provide adequate strength at that portion of the die. As a result, two or more turns of the thread at its inner end usually will be incomplete, having less than the full cross-sectional dimension of the remainder of the thread. The incomplete thread can carry no load, and hence does nothing to enhance the performance of the fastener. However, the fastener shank must be made sufficiently long to include the runout thread at its inner end, as well as the portion of the thread that carries the load. Typically, the transition section where the thread runout occurs has a length axially of the shank corresponding to twice the pitch of the thread.
Many fasteners include a nut or collar with a counterbore to receive the transition zone of the bolt or pin that includes the incomplete runout thread. It is necessary for the counterbore of the nut or collar to have a length sufficient to accommodate the runout thread. Therefore, the length of both the bolt and nut are dictated by the requirement for the incomplete thread at the inner end of the threaded portion of the bolt.
It has been recognized that making the pin or bolt shorter by reducing the length of the transition zone with its incomplete threads would result in a saving in weight of considerable significance in the aircraft and aerospace fields, as well as other areas where minimizing weight is critical. Nevertheless, conventional thread rolling will not permit this.
One approach to reducing the length of the thread runout has been to first form a groove in the transition zone to approximately the minor diameter of the thread to be produced. This is accomplished prior to rolling the thread, either by cutting to the required geometry or by cutting to less than full dimension, followed by cold rolling to produce a groove of the desired depth. After this, the thread is rolled, with the runout extending into the grooved portion. The result is a thread runout transition zone shorter than that of a conventional fastener with a rolled thread. A drawback to this system is the increased expense incurred in the extra operation of forming the groove in the shank prior to the thread-rolling operation.